Process for the production of high-octane hydrocarbon compounds by the selective dimerization of isobutene contained in a stream which also contains C5 hydrocarbons

ABSTRACT

A process is described for the production of high-octane hydrocarbon compounds by means of the selective dimerization of isobutene, in the presence of C 5  hydrocarbons and oxygenated compounds (branched alcohols or alternatively blends of linear or branched alcohols and alkyl ethers) characterized in that it utilizes a catalytic distillation as second reaction step.

The present invention relates to a process for the production ofhigh-octane hydrocarbon compounds by means of the selective dimerizationof isobutene and, to a lesser extent, of possible linear olefins, in thepresence of C₅ hydrocarbons and oxygenated compounds, which favour theformation of higher selectivities on the part of the catalyst.

The mixture obtained can then be hydrogenated with conventional methodsto obtain a product with further enhanced octane characteristics.

For mainly environmental reasons, the composition of gasolines is beingreformulated and the general tendency is towards the production of fuelswhich burn better and have lower evaporative emissions. The mainmeasures for achieving this objective are listed below (D. Sanfilippo,F. Ancillotti, M. Marchionna, Chim. & Ind., 76, (1994), 32):

-   -   reduction in the content of aromatic compounds and elimination        of benzene;    -   reduction in the volatility of gasolines to minimize evaporative        losses;    -   reduction in the content of light olefins, photochemically        extremely reactive;    -   reduction in the sulfur content and final boiling point of the        gasolines.

All these measures consequently create the necessity of projecting newproduction processes of purely hydrocarbon compounds capable ofpositively contributing to the above demands.

Among these, alkylated products are extremely important as they have ahigh octane number, a low volatility and are practically free of olefinsand aromatic compounds. The alkylation process in liquid phase is areaction between isoparaffinic hydrocarbons, such as isobutane, andolefins, for example propylene, butenes, pentenes and relative mixtures,in the presence of an acid catalyst for the production of C₇-C₉hydrocarbons with a high octane number to be used in gasolines (A.Corma, A. Martinez, Catal. Rev.—Sci. Eng., 35, (1993), 483).

The main problem of alkylation processes is due to the fact that, withgrowing environmental regulations, both of the traditional processes(with hydrofluoric and sulfuric acid) are encountering considerabledifficulties, which create uncertainties for the future; the processwith hydrofluoric acid due to the toxicity of this acid, especially inpopulated areas, and that using sulfuric acid, as a result of the largeproduction of acid sludge as well as the considerably corrosive natureof the catalyst.

Alternative processes with solid acid catalysts are being developed buttheir commercial applicability has yet to be demonstrated.

A hydrocarbon product of this type, on the other hand, is becomingincreasingly more requested due to its octane characteristics (both theResearch Octane Number (RON) and the Motor Octane Number (MON) are high)and those relating to the boiling point (limited volatility but lowend-point) which position it in the group of compositions of greatinterest for obtaining gasolines which are more compatible with currentenvironmental requirements.

An alternative refinery process for obtaining products withcharacteristics similar to those of alkylated products can be offered bythe hydrogenation of so-called “polymer” gasoline.

Oligomerization processes (often inaccurately called polymerization inthe refining industry) were widely used in the ‘30s’ and ‘40s’ forconverting low-boiling C₃-C₄ olefins into gasolines. The process leadsto the production of a gasoline with a high octane number (RON about 97)but with a high sensitivity (difference between RON and MON) due to thepurely olefinic nature of the product (J. H. Gary, G. E. Handwerk,“Petroleum Refining: Technology and Economics”, 3^(rd) Ed., M. Dekker,New York, (1994), 250).

Typical olefins which are oligomerized are mainly propylene, which givesdimers or slightly higher oligomers depending on the process used, andisobutene which mainly gives dimers but is always accompanied by aconsiderable quantity of higher oligomers.

With particular attention to the oligomerization of isobutene, it isknown that this reaction can be carried out either batchwise,semi-batchwise or in continuous, either in gas or liquid phase,generally at temperatures ranging from 50 to 300° C. and at atmosphericpressure or such pressures as to maintain the reagents in liquid phase,if necessary.

Typical catalysts for the industrial oligomerization process ofisobutene are represented by phosphoric acid, generally supported on asolid (for example kieselguhr), or cation-exchange acid resins. Thelatter allow blander conditions to be used compared with supportedphosphoric acid both in terms of temperature and pressure (50-100° C.and 0.2-3 MPa with respect to 200-220° C. and 3-10 MPa).

Other catalysts are also claimed in literature, both liquid acids suchas H₂SO₄ and derivatives of sulfonic acids, and solids such assilico-aluminas, mixed oxides, zeolites, fluorinated or chlorinatedaluminas, etc.; none of these catalysts however has so far enabled anindustrial process to be set up, as in the case of supported phosphoricacid (F. Asinger, “Mono-olefins: Chemistry and Technology”, PergamonPress, Oxford, pages 435-456) and that of cation resins (G. Scharfe,Hydrocarbon Proc., April 1973, 171).

From the product point of view, the main problem of this process lies inthe fact that excessive percentages of heavy oligomers such as trimers(selectivity of 20-40%) and tetramers (selectivity of 1-5%) ofisobutene, are produced in the oligomerization phase. Tetramers arecompletely outside the gasoline fraction as they are too high-boilingand therefore represent a net loss in yield to gasoline; as far astrimers are concerned, their concentration should be greatly reduced asthey have a boiling point (170-180° C.) at the limit of futurespecifications on the final point of reformulated gasolines.

The problem of reducing the formation of oligomers higher than dimers topercentages lower than 15% is, on the other hand, a problem typical ofthe oligomerization of isobutene, as also indicated in literature (C. T.O'Connor, M. Kojima, K. W. Shcumann, Appl. Catal., 16, (1985), 193).This level of heavy compounds is slightly higher than that of analkylated product and is still tolerated in the gasoline pool.

From what is specified above, there is evidently great interest inobtaining a new dimerization process of isobutene which allows thesynthesis of a higher-quality product, through reaching greaterselectivities.

By carrying out the selective dimerization reaction of isobutene in thepresence of moderate quantities of oxygenated products, the productionof a fraction of oligomers is selectively obtained, which is particularrich in dimers (>85%) and practically free of tetramers and higheroligomers (<0.5%).

The reaction product is then preferably hydrogenated to give acompletely saturated end-product with a high octane number and lowsensitivity.

The hydrogenation can be carried out with conventional methods asdescribed, for example, in F. Asinger, “Mono-olefins: Chemistry andTechnology”, Pergamon Press, Oxford, page 455.

SUMMARY OF THE INVENTION

One embodiment of the invention includes a process for producinghigh-octane hydrocarbon compositions by dimerizing isobutene in thepresence of C₅ hydrocarbons and oxygenated compounds utilizing acatalytic distillation as reaction step. In another embodiment thedimerizing is carried out in the presence of moderate quantities ofoxygenated products to form an oligomer fraction that is particular richin dimers and practically free of tetramers and higher oligomers.

In other embodiments the process includes producing high-octanehydrocarbon compounds by selectively dimerizing isobutene in mixturescomprising C₅ hydrocarbons such that (i) the reaction is carried out intwo distinct steps and (ii) a catalytic distillation is used as secondstep; and wherein the dimerizing is carried out in the presence ofoxygenated products selected from a branched alcohol alone or in a blendwith linear alcohols and alkyl ethers, in such a quantity as to have inthe feeding, in the case of the presence of a branched alcohol alone, amolar ratio oxygenated product/isobutene higher than 0.005, in the caseof the presence of a branched alcohol in a blend, a molar ratiooxygenated product/isobutene higher than 0.01.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process scheme with catalytic distillation where C₅products are not present in the charge and the oxygenated product is abranched alcohol;

FIG. 2 shows a process scheme including a catalytic distillation, tworeaction steps (fixed bed reactors) and three fractionation columns;

FIG. 3 shows a process scheme in which the C₅/TBA azeotropic product isrecovered from the head of a column;

FIG. 4 shows a process scheme with a fractionation column;

FIG. 5 shows a process scheme in which an C₅/TBA azeotropic product isrecovered as a side cut in a column reactor;

FIG. 6 shows a process scheme in which an azeotropic product is sent toa head stream of a catalytic distillation column;

FIG. 7 shows a process scheme that includes a mixture of oxygenatedproducts.

For illustrative purposes, Table 1 indicates the octane number andrelative boiling points of some of the products obtained, by means ofthe process, object of the present invention.

TABLE 1 Product RON MON b.p. (° C.) Diisobutenes 100 89 100-105Iso-octane 100 100 99 Tri-isobutenes 100 89 175-185 Hydrogenated 101 102170-180 tri-isobutenes

The process, object of the present invention, for the production ofhigh-octane hydrocarbon compounds by the selective dimerization ofisobutene contained in a stream also containing C₅ hydrocarbons, ischaracterized in that:

the reaction is carried out in two distinct steps,

a catalytic distillation is used as second step,

the reaction is carried out in the presence of oxygenated productsselected from a branched alcohol alone or in a blend with linearalcohols and alkyl ethers, in such a quantity as to have in the feeding,in the case of the presence of a branched alcohol alone, a molar ratiooxygenated product/isobutene higher than 0.005, in the case of thepresence of a branched alcohol in a blend, a molar ratio oxygenatedproduct/isobutene higher than 0.01.

It should also be pointed out that in the case of hydrocarbon streamsalso comprising other olefins, it has been observed that at least a partof the latter can be converted by reaction with isobutene into thehydrocarbon product without altering the octane value. It is thereforepreferable to effect an enriching treatment, by means ofpre-isomerization, of the internal linear olefins, in order to favourthe overall octane number of the mixture.

The process claimed herein can be applied to cuts mainly containingisobutane, isobutene, n-butane, n-butenes and saturated and olefinic C₅hydrocarbons.

Although a wide variety of sources are available for the supply of thesestreams, the most common are those deriving from Dehydrogenationprocesses of iso-paraffins, from FCC units, Steam Cracking or processesfor the production of pure isobutene such as the dehydration oftert-butyl alcohol (TBA) or the Cracking of MTBE and/or ETBE; thesestreams differ from each other in the content of isobutene and linearbutenes, as shown in Table 2.

TABLE 2 Steam Pure Cracking FCC Dehydrogenation isobutene Isobutene30-50 10-25 45-55 >90 n-butenes 35-60 25-50 1-2 <10 Saturated C₄  4-1030-60 45-55 <1

Should streams from Steam Cracking contain diolefins in addition to thedesired mono-olefins, they must be eliminated by means of typicalremoval treatment (for example solvent extraction or selectivehydrogenation).

Saturated and olefinic C₅ hydrocarbons can be present in these streams,in various amounts (0.2-20%), depending on the efficiency of the C₄-C₅separation step. The C₅ olefins possibly present can be involved indimerization reactions.

The stream sent to the reaction steps can contain branched alcohols or ablend of alcohols and alkyl ethers, in addition to the hydrocarboncomponents.

The linear alcohols used contain a number of carbon atoms ranging from 1to 6 and those preferred are methanol and/or ethanol. The branchedalcohols have from 3 to 6 carbon atoms and those preferred aretert-butyl alcohol (TBA) and/or tert-amyl alcohol (TAA).

The alkyl ether used can be selected from those containing a number ofcarbon atoms ranging from 5 to 10: MTBE (methyl tert-butyl ether), ETBE(ethyl tert-butyl ether), MSBE (methyl sec-butyl ether), ESBE (ethylsec-butyl ether), TAME (methyl tert-amyl ether), TAEE (ethyl tert-amylether) or mixtures thereof are preferred.

Isobutene, together with the hydrocarbon stream in which it iscontained, is sent with the oxygenated products, in stoichiometricdefect, into contact with the acid catalyst where the dimerization takesplace. The linear primary alcohol, possibly present, in addition tointeracting with the catalysts, also helps to limit the possiblecracking of the alkyl ether and can possibly react with the dimers andlinear C₄ olefins, whereas the branched alcohol (tertiary) does notreact with the olefins due to its steric hindrance.

In order to obtain the dimerization product with the desired selectivityto dimers, it is essential to maintain a constant level of oxygenatedproducts in the reaction environment to form the catalytic species withthe correct activity and stability. The optimal level of oxygenatedproducts present in the reaction environment, to obtain selectivities todimers close to 85% by weight, depends on the composition of thehydrocarbon charge. The higher the olefin content in the charge, thelower the amount of oxygenated products to be used.

A wide variety of acid catalysts can be used for this process, but thosepreferred are styrene-divinyl benzene polymeric resins having sulphonicgroups as catalytic centres.

A large range of operative conditions can be used to produce high-octanehydrocarbons from isobutene in the desired selectivities. It is possibleto operate in vapour or liquid-vapour phase, but operating conditions inliquid phase are preferred.

The pressure is preferably higher than the atmospheric value, in orderto maintain the reagents in liquid phase, generally below 5 MPa, morepreferably between 0.2-2.5 MPa. The reaction temperature preferablyranges from 30 to 120° C.

The feeding space velocities of the oxygenated-hydrocarbon stream arepreferably lower than 30 h⁻¹, more preferably ranging from 1 and 15 h⁻¹.

Isobutene is mainly converted in the reaction zone, however portions ofthe other paraffins which are present can also be converted to usefulproduct; in principle, there are no limits to the concentration ofiso-olefin in the hydrocarbon fraction, even if concentrations rangingfrom 2 to 60% are preferred; in case of streams having a high isobuteneconcentration (dehydration or cracking) it is therefore convenient todilute the charge with C₄-C₇ hydrocarbons. There are no limits, on thecontrary, for the ratio between isobutene and linear olefins.

The process, object of the present invention, can be effected batchwiseor in continuous, bearing in mind however that the latter is much moreadvantageous in industrial practice.

The reactor configuration selected includes a first reaction step (oneor more fixed bed reactors) and a second step consisting of a catalyticdistillation which avoids the use of a reactor and a distillationcolumn, as in a conventional plant.

The presence of C₅ hydrocarbons in the feed, however, complicates theprocess schemes, as these compounds have intermediate boilingtemperatures between C₄ and oxygenated products, and they also formazeotropic mixtures with the branched alcohols as shown in Table 3,which indicates the boiling points of the most representativelow-boiling components present in the streams

TABLE 3 Compound Boiling point, ° C. C₄/Methanol azeotropic product −5C₄ products −12/1 Isopentane/TBA azeotropic product 25 Isopentane 281-pentene 30 2-methyl-1-butene 31 n-pentane 36 2-methyl-2-butene 39cyclopentane 49 MTBE 55 Methanol 65 Dimers/TBA azeotropic product 78 TBA82 Dimers 100-105

The C₅ products cannot therefore be removed from the plant together withthe C₄ products, as they would introduce oxygenated products (branchedalcohols) into the stream, which are difficult to remove by means of thetraditional techniques used for removing methanol (water washing) andwhich are poisonous for the subsequent treatment processes of thestreams (polymerization, alkylation and metathesis).

The C₅ products, on the other hand, cannot be maintained in theoxygenated stream as they would rapidly accumulate. With respect to theschemes shown in literature (U.S. Pat. No. 6,011,191), it is thereforenecessary to introduce a C₅/branched alcohol azeotropic separation step,which can be inserted in several positions of the plant, in relation tothe C₅ content in the charge and also the relative concentration of theC₅ products present.

When the oxygenated product is a branched alcohol alone, the process is,in particular, preferably effected with a molar ratio of oxygenatedproduct/isobutene lower than 0.6, through the following essential steps:

-   a) feeding the C₄-C₅ hydrocarbon cut containing isobutene to the    first reaction step (consisting of one or more reactors), together    with one or more streams containing oxygenated products;-   b) using a catalytic distillation column as second reaction step,    wherein the isobutene conversion is completed, in addition to the    separation of the reagents/products;-   c) recovering the C₅ hydrocarbon/branched alcohol azeotropic    product, in one or more fractionation columns, also catalytic, as    head stream, side cut or bottom stream;-   d) recycling the stream containing the oxygenated products and    possibly the reintegrated oxygenated products, to the two reaction    steps;-   e) possibly recycling part of the C₄ products to the first reaction    step, in order to maximize the isobutene conversion.

The first reaction step can consist of one or more fixed bed, tubularand/or adiabatic reactors.

The separation of the C₅/branched alcohol azeotropic product of step (c)is preferably effected starting from blends:

-   -   a) C₅—oxygenated products—reaction product, wherein the C₅        hydrocarbons are recovered as azeotropic compound with the        branched alcohol, as head effluent, using a scheme based on one        or two fractionation columns;    -   b) C₄-C₅—oxygenated products—reaction product, wherein the C₅        hydrocarbons are recovered as azeotropic compound with the        branched alcohol as side cut of a catalytic distillation column        from whose head the C₄ products are recovered and at the bottom        a blend containing the oxygenated products and the reaction        product;    -   c) C₄-C₅—oxygenated products, wherein the C₅ hydrocarbons are        recovered as azeotropic compound with the branched alcohol as        the bottom effluent of a fractionation column from whose head        the C₄ products are recovered.

When the oxygenated product is a branched alcohol in a mixture withlinear alcohols and alkyl ethers, the process is preferably effected, inparticular, with a molar ratio of oxygenated product/isobutene lowerthan 0.7, by means of the following essential steps:

-   -   a) feeding the C₄-C₅ hydrocarbon cut containing isobutene to the        first reaction step (consisting of one or more reactors),        together with one or more streams containing oxygenated products        (linear and branched alcohols, ethers and water);    -   b) using a catalytic distillation column as second reaction        step, wherein the isobutene conversion is completed, in addition        to the separation of the reagents/products;    -   c) separating the C₄/linear alcohol azeotropic product and        possibly C₄ products from the remaining oxygenated compounds and        from the hydrocarbon product, in one or more distillation        columns, also catalytic;    -   d) recovering the linear alcohol from the azeotropic product        with the C₄ compounds, by means of conventional processes such        as water washing or adsorption on inorganic solids;    -   e) recovering the C₅/branched alcohol azeotropic product, in one        or more fractionation columns, also catalytic, as head stream,        side cut or bottom stream;    -   f) recycling the stream containing the oxygenated products        (branched alcohol and ether) and possibly the reintegrated        oxygenated products and recovered linear alcohol, to the two        reaction steps;    -   g) possibly recycling part of the C₄ products to the first        reaction step, in order to maximize the isobutene conversion.

The first reaction step can consist of one or more adiabatic reactors,such as traditional, boiling point, expanded bed reactors.

The separation of the C₅/branched alcohol azeotropic product of step (e)is preferably effected starting from blends of:

-   -   a) C₅—oxygenated products (ethers and branched        alcohols)—reaction product, wherein the C₅ hydrocarbons are        recovered as an azeotropic compound with the branched alcohol,        as head effluent, using a scheme based on one (recovery of the        remaining oxygenated products as side cut) or two fractionation        columns;    -   b) C₅—oxygenated products (ethers and branched alcohols—dimers,        in which the C₅ hydrocarbons are recovered as azeotropic product        with the branched alcohol as head effluent of a fractionation        column;    -   c) C₄-C₅—oxygenated products (ethers and linear and branched        alcohols)—reaction product, effluent from a reaction step,        wherein the C₅ hydrocarbons are recovered as an azeotropic        compound with the branched alcohol as side cut of a        fractionation column from whose head the C₄/linear alcohol        azeotropic product and possibly the C₄ products are recovered,        whereas a mixture containing the oxygenated products and the        reaction product is recovered at the bottom;    -   d) C₄-C₅—oxygenated products (linear and branched alcohols)        wherein the C₅ hydrocarbons are recovered as an azeotropic        compound with the branched alcohol as bottom effluent of a        fractionation column from whose head the C₄/linear alcohol        azeotropic product and possibly C₄ products are recovered.

For the two processes comprising the essential steps specified above(a-e and a-g) the C₅ products are present in the streams prevalentlycontaining C₄ products in a quantity preferably ranging from 0.5 to 10%by weight.

Seven process schemes are shown in FIGS. 1-7, in order to clearlyillustrate the present invention.

FIG. 1 shows a process scheme with catalytic distillation, when C₅products are not present in the charge and the oxygenated product is abranched alcohol (TBA).

The stream (1) containing isobutene, together with the reintegrationfeeding of TA (or possibly water) (2) and the recycled stream ofoxygenated products (9), is sent to a first reaction step (R1), whichcan consist of one or more fixed bed reactors, in which the C₄iso-olefin is selectively converted to dimers.

The effluent (4) from the first reaction step, is sent to a catalyticdistillation (C1) in which the isobutene conversion is completed. Astream (5) essentially containing C₄ hydrocarbons is removed from thehead of this column, whereas a stream (6) essentially containing thereaction product and the oxygenated compounds, is collected at thebottom.

This stream (6) is sent to a further separation column (C2) wherein astream (8) is collected at the head, containing the dimers/TBAazeotropic product which is recycled to the two reaction steps (streams9 and 10), whereas the reaction product (7) essentially consisting ofdimers and trimers, is collected from the bottom.

The introduction of catalytic distillation allows a considerablesimplification of the plant scheme, which is instead based on tworeaction steps (fixed bed reactors) and three fractionation columns, asshown in FIG. 2.

When C₅ hydrocarbons are present in the charge, different plantconfigurations can be used to recover the C₅/TBA azeotropic product,depending on the quantity of C₅ products present and the required purityof the streams.

FIG. 3 shows a possible process scheme which differs from that of FIG. 1due to the fact that the C₅/TBA azeotropic product (11) is recoveredfrom the head of the column C2, which can possibly be joined to thereaction product, whereas the stream containing the oxygenated productsto be recycled (TBA/dimers azeotropic product) is removed from thecolumn C2 as side cut (8).

The process scheme is more complex when a more efficient separation ofthe two C₅/TBA and dimers/TBA azeotropic products is to be effected, asa new fractionation column (C3) must be inserted, as shown in FIG. 4. Inthis new scheme, the head stream of the column C2 (8) is sent to a newcolumn (C3) wherein the C₅/TBA azeotropic product (11) is separated atthe head and the dimers/TBA azeotropic product (12) which is recycled tothe two reaction steps (streams 9 and 10), is separated at the bottom.

Alternatively, the C₅/TBA azeotropic product can be recovered as sidecut (11) in the column reactor C1 (FIG. 5).

A further option, shown in FIG. 6, consists in sending this azeotropicproduct to the head stream of the catalytic distillation column togetherwith the C₄ products and in using a new column (C3) to recover theC₅/TBA azeotropic product at the bottom (11) and C₄ products at the head(12).

FIG. 7 shows a possible process scheme when a mixture of oxygenatedproducts, consisting of alkyl ether (MTBE), linear alcohol (Methanol)and branched alcohol (TBA), is used. In this case, the stream (1)containing isobutene, together with the reintegration feeding ofmethanol and TBA (or water) (2) and the recycled streams of oxygenatedproducts (MTBE and TBA) (9) and methanol (14), is sent to the firstreaction step (R1), which can consist of one or more reactors, in whichthe C₄ iso-olefin is selectively converted to dimers.

The effluent (4) from the first reaction step is sent to catalyticdistillation (C1), which represents the second reaction step, together,possibly, with the recycled streams of oxygenated products (10) andmethanol (13). A stream (5) is collected from the head of this column,essentially containing C₄ hydrocarbons and methanol, which is fed to aunit for the recovery of the alcohol (MR) which can consist, forexample, of an adsorption system on molecular sieves, or a water washingcolumn. In both cases, the alcohol recovered (12) can be sent back tothe two reaction steps (streams 13 and 14), whereas the hydrocarbonstream (11) can be used in subsequent operations.

The bottom stream (6) of the column C1 is sent to a further separationcolumn (C3) wherein a stream (15) containing the C₅/branched alcoholazeotropic product is collected at the head, a stream (8) essentiallycontaining MTBE, TBA and dimers, as side cut, which is recycled to thetwo reaction steps (streams 9 and 10), whereas the reaction product (7),essentially consisting of dimers, trimers and small quantities ofoligomers is recovered from the bottom.

The invention claimed is:
 1. A process for the production of high-octanehydrocarbon compounds comprising a selective dimerization of isobutenecomprised in a stream also comprising C₅ hydrocarbons, wherein: theselective dimerization is carried out with acid catalysts in twodistinct steps wherein the first step consists of contacting the streamwith one or more fixed bed reactors and the second step consists ofcatalytically distilling the stream; wherein the selective dimerizationis carried out in the presence of one or more oxygenated productsselected from a branched alcohol alone or in a blend with linearalcohols and alkyl ethers, wherein the branched alcohol forms anazeotropic mixture with the C₅ hydrocarbons, in such a quantity as tohave in the feeding, in the case of the presence of a branched alcoholalone, a molar ratio of oxygenated product/isobutene higher than 0.005,in the case of the presence of a branched alcohol in a blend, a molarratio of oxygenated product/isobutene higher than 0.01; and aC₅/branched alcohol azeotropic separation step is carried out.
 2. Theprocess according to claim 1, wherein the first reaction step is carriedout at a reaction temperature ranging from 30 to 120° C., at a pressurelower than 5 MPa and feeding space velocities lower than 30 h⁻¹.
 3. Theprocess according to claim 2, wherein the feeding space velocities rangefrom 1 to 15 h⁻.
 4. The process according to claim 1, wherein thebranched alcohol has a number of carbon atoms ranging from 3 to
 6. 5.The process according to claim 4, wherein the branched alcohol isselected from tert-butyl alcohol or tert-amyl alcohol.
 6. The processaccording to claim 1, wherein water is fed to the reaction, the waterbeing capable of forming the branched alcohol by reacting with thetertiary olefin under the reaction conditions.
 7. The process accordingto claim 1, wherein the linear alcohol has a number of carbon atomsranging from 1 to
 6. 8. The process according to claim 7, wherein thelinear alcohol is selected from methanol and/or ethanol.
 9. The processaccording to claim 1, wherein the alkyl ether has a number of carbonatoms ranging from 5 to
 10. 10. The process according to claim 9,wherein the alkyl ether is selected from MTBE, ETBE, MSBE, ESBE, TAME,TAEE or mixtures thereof.
 11. The process according to claim 1, whereinthe oxygenated product is the branched alcohol alone, comprising: a)feeding a C₄-C₅ hydrocarbon cut comprising isobutene to the firstreaction step (consisting of one or more reactors), together with one ormore streams comprising oxygenated products; b) using a catalyticdistillation column as second reaction step, wherein isobuteneconversion is completed, in addition to separation of reagents/products;c) recovering a C₅ hydrocarbon/branched alcohol azeotropic product, inone or more catalytic fractionation columns, as head stream, side cut orbottom stream; d) recycling the stream comprising the oxygenatedproducts and optionally comprising the reintegrated oxygenated products,to the two reaction steps; and e) optionally, recycling part of the C₄products to the first reaction step, in order to maximize the isobuteneconversion.
 12. The process according to claim 11, wherein therecovering of the C₅/branched alcohol azeotropic product can be effectedstarting from blends of: a. C₅-oxygenated products—reaction product,wherein the C₅ hydrocarbons are recovered as an azeotropic compound withthe branched alcohol, as head effluent, using a scheme based on one ortwo fractionation columns; b. C₄-C₅-oxygenated products—reactionproduct, wherein the C₅ hydrocarbons are recovered as an azeotropiccompound with the branched alcohol as side cut of a catalyticdistillation column from whose head C₄ products are recovered and fromthe bottom a blend containing the oxygenated products and the reactionproduct; c) C₄-C₅-oxygenated products, wherein the C₅ hydrocarbons arerecovered as an azeotropic compound with the branched alcohol, as bottomeffluent of a fractionation column from whose head the C₄ products arerecovered.
 13. The process according to claim 11, wherein the firstreaction step is carried out in at least one of a fixed bed reactor, atubular reactor and an adiabatic reactor.
 14. The process according toclaim 11, wherein a molar ratio of oxygenated product/isobutene is lowerthan 0.6.
 15. The process according to claim 1, wherein the oxygenatedproduct is the branched alcohol in a blend with linear alcohols andalkyl ethers, comprising: a) feeding a C₄-C₅ hydrocarbon cut comprisingisobutene to the first reaction step, together with one or more streamscomprising the oxygenated products; b) using a catalytic distillationcolumn as second reaction step, wherein isobutene conversion iscompleted in addition to a separation of reagents/products; c)separating C₄ products and C₄/linear alcohol azeotropic products fromthe C₅ hydrocarbons, from remaining oxygenated compounds and from thehydrocarbon product, in one or more catalytic distillation columns; d)recovering the linear alcohol from the azeotropic product with the C₄compounds, by means of conventional processes such as water washing oradsorption on inorganic solids; e) recovering the C₅/branched alcoholazeotropic product, in one or more catalytic fractionation columns, ashead stream, side cut or bottom stream; f) recycling the streamcomprising the oxygenated products (branched alcohol and ether) andoptionally comprising the reintegrated oxygenated products and thelinear alcohol recovered, to the two reaction steps; g) optionally,recycling part of the C₄ products to the first reaction step to maximizethe isobutene conversion.
 16. The process according to claim 15, whereinthe recovering of the C₅/branched alcohol azeotropic product can beeffected starting from blends of: a) C₅-oxygenated products (ethers andbranched alcohols)—reaction product, wherein the C₅ hydrocarbons arerecovered as an azeotropic compound with the branched alcohol, as headeffluent, using a scheme based on one (recovery of the remainingoxygenated products as side cut) or two fractionation columns; b)C₅-oxygenated products (ethers and branched alcohols)—dimers, in whichthe C₅ hydrocarbons are recovered as an azeotropic product with branchedalcohol as head effluent of a fractionation column; c) C₄-C₅-oxygenatedproducts (ethers and linear and branched alcohols)—reaction product,effluent from a reaction step, wherein the C₅ hydrocarbons are recoveredas an azeotropic compound with the branched alcohol as side cut of afractionation column from whose head the C₄/linear alcohol azeotropicproduct and optionally the C₄ products are recovered, whereas a mixturecontaining the oxygenated products and the reaction product is recoveredat the bottom; d) C₄-C₅-oxygenated products (linear and branchedalcohols) wherein the C₅ hydrocarbons are recovered as an azeotropiccompound with the branched alcohol as bottom effluent of a fractionationcolumn from whose head the azeotropic product C₄/linear alcohol andoptionally C₄ products are recovered.
 17. The process according to claim15, wherein the first reaction step is carried out in one or more of atraditional adiabatic reactor, a boiling point adiabatic reactor, and anexpanded bed adiabatic reactor.
 18. The process according to claim 15,wherein a molar ratio of oxygenated product/isobutene is lower than 0.7.19. The process according to claim 11, wherein the C₅/branched alcoholazeotropic product is mixed with the reaction product.
 20. The processaccording to claim 1, wherein, in the case of concentrated isobutenestreams, a charge is diluted with C₄-C₇ hydrocarbons.
 21. A process forproducing a high-octane hydrocarbon composition, comprising: dimerizingisobutene present in a hydrocarbon stream comprising one or more C₅hydrocarbons by contacting the hydrocarbon stream with one or more acidcatalysts in one or more fixed bed reactors to form a dimerizedhydrocarbon stream comprising one or more C₅/branched alcoholazeotropes; then catalytically distilling the dimerized hydrocarbonstream to form a catalytically distilled hydrocarbon stream; thenseparating a C₅/branched alcohol azeotrope from the catalyticallydistilled hydrocarbon stream; wherein the hydrocarbon stream isdimerized in the presence of one or more oxygenated products selectedfrom the group consisting of a branched alcohol, a blend of a branchedalcohol with one or more linear alcohols, a blend of a branched alcoholwith one or more alkyl ethers, and a blend of a branched alcohol withone or more linear alcohols and one or more alkyl ethers, wherein thehydrocarbon stream comprises the isobutene and the oxygenated product inan oxygenated product/isobutene molar ratio of greater than 0.005 whenthe oxygenated product consists of a branched alcohol; or thehydrocarbon stream comprises the isobutene and the oxygenated product inan oxygenated product/isobutene molar ratio of greater than 0.01 whenthe oxygenated product is a blend of the branched alcohol with one ormore of the linear alcohol and the alkyl ether.
 22. The processaccording to claim 21, wherein the hydrocarbon stream is dimerized at atemperature of 30-120° C.; at a pressure of lower than 5 MPa; and at afeeding space velocity of lower than 30 h⁻¹.
 23. The process accordingto claim 21, wherein the branched alcohol has from three to six carbonatoms.
 24. The process according to claim 21, wherein the branchedalcohol is at least one selected from the group consisting of tert-butylalcohol and tert-amyl alcohol.
 25. The process according to claim 21,further comprising: feeding water to the stream during the dimerizing,and forming the branched alcohol by reacting a tertiary olefin withwater during the dimerizing.
 26. The process according to claim 21,wherein the oxygenated product comprises one or more linear alcoholshaving from one to six carbon atoms.
 27. The process according to claim21, wherein the oxygenated product comprises at least one linear alcoholselected from the group consisting of methanol and ethanol.
 28. Theprocess according to claim 21, wherein the oxygenated product comprisesone or more alkyl ethers having from five to ten carbon atoms.
 29. Theprocess according to claim 21, wherein the oxygenated product comprisesone or more alkyl ethers selected from the group consisting of MTBE,ETBE, MSBE, ESBE, TAME and TAEE.
 30. The process according to claim 21,wherein the oxygenated product consists of a branched alcohol and theprocess comprises: a) feeding a C₄-C₅ hydrocarbon cut comprisingisobutene to a fixed bed reactor together with one or more other streamscomprising the oxygenated product during the dimerizing; b) convertingthe isobutene and the C₄-C₅ hydrocarbon cut during the catalyticdistilling and then separating the converted C₅-C₄ hydrocarbon cut; c)separating the C₅ hydrocarbon/branched alcohol azeotrope in one or morecatalytic fractionation columns as at least one of a headstream, a sidecut and a bottom stream; d) recovering the C₅ hydrocarbon/branchedalcohol azeotrope separated in (c) and, optionally, recycling one ormore reintegrated oxygenated products to the dimerizing and thecatalytically distilling; and e) optionally recycling a portion of a C₄dimerization product to the dimerizing.
 31. The process according toclaim 30, wherein the recovering meets at least one of the followingconditions: (a) the C₅/branched alcohol azeotrope is recovered as a headeffluent from one or two fractionation columns; (b) the C₅hydrocarbon/branched alcohol azeotrope is recovered as a side cut of acatalytic distillation column producing a C₄ head product and a bottomproduct comprising a blend of the oxygenated products and thecatalytically distilled product; and (c) the C₅ hydrocarbon/branchedalcohol azeotrope is recovered as a bottom effluent of a fractionationcolumn having a head forming a C₄ product.
 32. The process according toclaim 21, wherein the oxygenated product comprises a blend of thebranched alcohol, the linear alcohol and the alkyl ether, and whereinthe process comprises: (a) feeding a C₄-C₅ hydrocarbon cut comprisingisobutene to the selective dimerization in combination with one or morestreams comprising the oxygenated product; b) converting the isobuteneand the C₄-C₅ hydrocarbon cut during the catalytic distilling and thenseparating the converted C₅-C₄ hydrocarbon cut; c) separating the C₅hydrocarbon/branched alcohol azeotrope in one or more catalyticfractionation columns as at least one of a headstream, a side cut and abottom stream; (d) recovering the linear alcohol is from the C₄/linearalcohol azeotrope; (e) recovering the C₅ hydrocarbon/branched alcoholazeotrope in one or more catalytic fractionation columns as a least oneof a head stream, a side cut or a bottom stream; and (f) recycling theoxygenated products to at least one of the catalytic distillation andthe selective dimerization.